In a horizontal vibrating screen using either a straight line reciprocating action or my exclusive oval action, disclosed and claimed in U.S. Pat. No. 3,442,381, it is very valuable to be able to vary the stroke angle relative to the horizontal. A steep angle, say 60.degree., will retain the materal being screened for a longer time on the screening surface and thereby increase the screening accuracy, but at the cost of reduced capacity moving over the screen. A flatter angle of say 30.degree. will move the material rapidly, but with less screening accuracy. Most screens, other than that disclosed in my above-mentioned patent, operate on a fixed-at-factory stroke angle or can only be changed with much difficulty. One gear must be removed from its shaft, the number of teeth counted and divided into 360.degree. to get the number of degrees per tooth, then replaced with the desired degree change. Gears must be tight on their shafts or troubles will occur. It is readily understood that this is a difficult, cumbersome and time consuming task. It is seldom that anyone will bother doing so once the machine is in service. Moreover, the vibrating mechanism must be at or very near the centroid (center of gravity) of the screen box to maintain uniform action should timing be varied.
Because of the unique design of my three shafted screen disclosed in my above-mentioned U.S. Pat. No. 3,442,381, it has been found to be possible to mount the planes of gears directly over centers of self-aligning bearings. This is a very important and valuable feature because gear centers will not change should a timing shaft fail or become loose in its hub.